1. Technical Field
The present disclosure relates to an electronic power supply circuit of a load composed of at least two independent circuit branches connected in parallel to each other, and in particular where a first branch may require a different power supply voltage from the second circuit branch. The proposed power supply circuit is particularly suitable for driving LEDs, for example white light LEDs for back-lighting.
2. Description of the Related Art
White-light LEDs are the most widely used components for the back-lighting of mobile phones. The back-lighting of screens and keypads requires an increasingly intense white light to ensure the high quality of multimedia applications, for example in mobile phones.
Such applications require extremely accurate control of the conduction current of the LEDs so as to obtain a clean, clear light and high light intensity with the minimum number of external components for the control of the LEDs.
The precision of current control requires technical solutions based on serial connection of the LEDs. To power the LEDs connected in series, an architecture based on a DC-DC converter (also known as a “switched-mode power supply”, SMPS) of the Step-Up type is used.
To reduce the number of external components, the SMPS are adapted to the specific application so as to have a single output and several feedback inputs, one for each circuit branch regardless of the LEDs being controlled.
However, the known architectures are not free of flaws. Specifically, current architectures based on SMPS step-up with voltage or current control, used to regulate the load current, do not permit optimized performance.
Some embodiments of such circuit architectures according to the known art will now be described.
FIG. 1 shows an embodiment solution with output voltage control. The dotted block shows the converter step-up circuit (SMPS). A voltage divider (R1, R2) is used to define the output voltage of the converter powering the two branches of LEDs connected in parallel. In order to control the current of each branch, a resistor (R) is added in series with the LEDs. The resistor sets the current that must circulate in each branch. However, the value of the current is not predictable, in that it depends on the voltage output and on the number “n” of LED diodes present in each branch. The value of the resistor R is set by:R=(Vout−nVled)/Iled where Iled is the current flowing in each branch.
The difference in Vled voltage depends on the conduction current of the diode and on the tolerance of the technological production process. For this reason, the correct value of the resistor R must be chosen case by case, so as to compensate the maximum voltage variations.
The output voltage is a design parameter and is set a priori to power each branch. The designer must consider the worst case of possible differences between the LEDs so as to ensure the necessary output voltage level Vout to light the LEDs. This means oversizing the circuit and therefore not being able to optimize the architecture. Such situation clearly becomes even more critical in the presence of a different number of LEDs on each branch.
FIG. 2 shows an architecture with current control. In this case, maintaining the parallel connection between the branches of LEDs, the circuit no longer sets the output voltage. This is set by the number of LEDs connected in series. The current circulating in the branch closest to the converter (master branch) is used to close the feedback loop on the converter. The branch further away (slave branch) is not controlled by the loop; its current value may be set by regulating a resistor in series (R).
Such solution presents two drawbacks: the precision in controlling the current in the slave branch and control in the case of a different number of LEDs. In fact, the circuit controls the master branch only and has no return information from the other (slave) branch. The risk is that there may be no current flow in the LEDs not regulated directly. This situation could arise when the master branch has a lower number of LEDs than the other branch or when there are the same number of LEDs but the uncontrolled branch needs a higher power voltage than the regulated branch (on account of the tolerances of the LEDs mentioned above).
FIG. 3 shows another architecture, designed to resolve the problem of managing a different number of LEDs on two branches. In order to guarantee all the branches in parallel the voltage needed to light the LEDs, the use of a voltage control circuit is required. The required voltage is set by step-step regulation. The value of the resistor R2 is regulated (increased or decreased) depending on the Vout voltage needed to guarantee the current in each branch. As a result, the output voltage Vout is regulated dynamically so as to choose the correct value.
Even though this architecture seems to resolve the problems of this application, a drawback remains in any case. The limitation of this solution is that efficiency is not optimized in the case of a different number of LEDs in each branch. In fact, the branch with a lower number of LEDs regulates the current but could determine an excessive dissipation of power (regulating the current all the time).
FIG. 4a shows a different architecture with current control mode. In this case, more than one node is used for feedback control, one for each branch.
In this solution, a timer circuit is provided which generates for example two digital square wave synchronized signals outphased so that they are never at the logic level “1” at the same time (FIG. 4b). Such timer circuit instructs the converter to supply current at two loads in “time sharing”, in other words alternately, even though such power alternation cannot be visually perceived by the user. The voltage output Vout is set on the basis of the number of LEDs connected in series in each branch. In this case there is no master branch or slave branch. This solution therefore resolves the problem of precise control of the current in both branches. In fact, the voltage output Vout is adapted alternately, depending on the timer signals, to the value requested by the regulation loop. The drawback of this solution however is the high level of overcurrent created in each branch during commutation. The peak current is particularly high in the case of different numbers of LEDs in each branch.
FIGS. 5a-5b show the diagrams of the timer signals of the voltage and of the output currents in the case of the circuit in FIG. 4a. In such diagrams Vout1 and I1 indicate the voltage and the output current relative to the branch with four LEDs; Vout2 and I2 indicate the voltage and output current relative to the branch with two LEDs. Clearly the output voltage Vout1 for the first branch must be greater than the output voltage Vout2 of the second branch, which has a lower number of LEDs.
FIG. 5b shows the same wave forms as FIG. 5a obtained using an oscilloscope.
The time required to pass from the output voltage Vout1 to the lower output voltage Vout2 depends on the value of the output capacity, on the value of the current I2 circulating in the second branch and on the speed of the feedback loop.
As can be seen from FIGS. 5a and 5b, the current I2 in the second branch initially shows a very high peak, due to the fact that the output capacity of the converter does not discharge immediately. In other words, during the transition and until the output capacity discharges, at the ends of the second branch there is a much higher power supply voltage than is needed to control the LEDs of the second branch.
The current peak constitutes a serious problem for the reliability of LEDs.